Parallel circuit feed with different raceways

[Coastalguy1]

I have a piece of equipment (HVAC) that is getting upgraded. The existing circuit is (3)#1/0 THHN cu,(1)#6 cu gnd in 1-1/4" EMT conduit.
My new equipment's MCA=165amps. If I parallel a (3) conductor 1/0 THHN cu w/gnd MC cable next to it I can achieve and meet the 165amps MCA for the new equipment. Choice for MC Cable vs. EMT is cost being it's a run across a building ceiling space. Voltage drop has been calculated.
My question is can I have a parallel feed with different raceway types?

 

[jeremy.zinkofsky]

I have a piece of equipment (HVAC) that is getting upgraded. The existing circuit is (3)#1/0 THHN cu,(1)#6 cu gnd in 1-1/4" EMT conduit.
My new equipment's MCA=165amps. If I parallel a (3) conductor 1/0 THHN cu w/gnd MC cable next to it I can achieve and meet the 165amps MCA for the new equipment. Choice for MC Cable vs. EMT is cost being it's a run across a building ceiling space. Voltage drop has been calculated.
My question is can I have a parallel feed with different raceway types?

Raceway material shouldn't be a factor as long as you meet the requirements of 310.10(H) & 250.122(F).

 

[iwire]

I have a piece of equipment (HVAC) that is getting upgraded. The existing circuit is (3)#1/0 THHN cu,(1)#6 cu gnd in 1-1/4" EMT conduit.
My new equipment's MCA=165amps. If I parallel a (3) conductor 1/0 THHN cu w/gnd MC cable next to it I can achieve and meet the 165amps MCA for the new equipment. Choice for MC Cable vs. EMT is cost being it's a run across a building ceiling space. Voltage drop has been calculated.
My question is can I have a parallel feed with different raceway types?

No, you cannot mix cable and raceway types they must have the same characteristics

You will also have issues with 250.122(F) which requires a full size EGC with each parallel set.

 

[jumper]

No, you cannot mix cable and raceway types they must have the same characteristics

How does the type of raceway effect the characteristics of a conductor in regards to a parallel install?

Nothing I see in 310.10(H) mentions raceway type.

 

[jeremy.zinkofsky]

How does the type of raceway effect the characteristics of a conductor in regards to a parallel install?

Nothing I see in 310.10(H) mentions raceway type.

I thought the same thing but looked at 310.10(H)(3). It is pretty vague but you could interpret it as requiring the raceway to have the same electrical characteristics.

 

[jumper]

I thought the same thing but looked at 310.10(H)(3). It is pretty vague but you could interpret it as requiring the raceway to have the same electrical characteristics.

But 310.10(H)(2) defines the characteristics required for the conductors, not the raceway, required in 310.10(H)(3).

 

[jeremy.zinkofsky]

But 310.10(H)(2) defines the characteristics required for the conductors, not the raceway, required in 310.10(H)(3).

310.10(H)(3):

"Where run in seperate cables or raceways, the cables or raceways with conductors shall have the same number of conductors and shall have the same electrical characteristics."

It clearly states that the cables or raceways shall have the same characterisitcs. But, like I said, it could be up for interpretation.

310.10(H)(2) does define the characteristics required for conductors and 310.10(H)(3) then defines the characteristics of the cables and raceways used when connecting conductors in parallel.

 

[Phil Corso]

Jeremy...

No! It is not a matter of interpretation! Iwire and Jumper make the point. While their is no difference in their resistance's, there is in their reactance's! A circulating-current will be produced, resulting in additional heating.

Regards, Phil Corso

 

[iwire]

How does the type of raceway effect the characteristics of a conductor in regards to a parallel install?

Nothing I see in 310.10(H) mentions raceway type.

If you used say PVC for one and RMC for another the current will not split evenly.

 

[jeremy.zinkofsky]

Jeremy...

No! It is not a matter of interpretation! Iwire and Jumper make the point. While their is no difference in their resistance's, there is in their reactance's! A circulating-current will be produced, resulting in additional heating.

Regards, Phil Corso

Where is that stated explicitly in the code sections that have been discussed? All physics aside; In regards to the original question, and only in what is required by the NEC, can you run a parallel feed in two different types of raceways?

 

[iwire]

In regards to the original question, and only in what is required by the NEC, can you run a parallel feed in two different types of raceways?

A metal conduit and a MC cable assembly will not have the same electrical characteristics.

I am sure you know this but it takes very little differences in characteristics to produce a large difference in how the current would split during a fault.

 

[jeremy.zinkofsky]

A metal conduit and a MC cable assembly will not have the same electrical characteristics.

I am sure you know this but it takes very little differences in characteristics to produce a large difference in how the current would split during a fault.

I do not disagree at all and am aware of the underlying physics, but we are just talking about the NEC here. And to that regard, would you agree that 310.10(H)(3) requires the raceways of a separate run of parallel conductors to have the same electrical characterisitcs?

 

[Phil Corso]

Jeremy...

Have I correctly interpreted your interpretation of "physical characteristics" to exclude consideration of electrical parameters that are altered as it applies to raceway or sheath materials, i.e., magnetic or non-magnetic?

Phil

 

[don_resqcapt19]

In the 2005 code it said:

Where run in separate raceways or cables, the raceways or cables shall have the same physical characteristics.

But in the 2008 code it says:

Where run in separate cables or raceways, the cables or raceways with conductors shall have the same number of conductors and shall have the same electrical characteristics.

The proposal, 6-7a, that made the change for the 2008 code was a panel 6 proposal and there is no clear information as to why the wording was changed from physical characteristics to electrical characteristics.

Clearly the panel believes there is a difference, but I have no idea what they were driving at.

Other rules cover the electrical characteristics of the conductors themselves and I think had they left the wording as it was in 2005, it would have been much clearer that you could not mix conduit types or conduits and cables in a parallel application.

 

[Phil Corso]

Gentlepeople...

The difference is shown in NEC, Chap 9, Table 9, "Alternating-Current Resistance and Reactance... "! Note, specifically, the 2nd column, "XL (Reactance) for All Wire)"! It lists the impact of raceway property (magnetic or not) on a major electrical characteristic, wire reactance!

Phil

 

[iwire]

And to that regard, would you agree that 310.10(H)(3) requires the raceways of a separate run of parallel conductors to have the same electrical characterisitcs?

2014 NEC 310.10(H)

(3) Separate Cables or Raceways. Where run in separate
cables or raceways, the cables or raceways with conductors
shall have the same number of conductors and shall have
the same electrical characteristics. Conductors of one
phase, polarity, neutral, grounded circuit conductor, or
equipment grounding conductor shall not be required to
have the same physical characteristics as those of another
phase, polarity, neutral, grounded circuit conductor, or
equipment grounding conductor.

In my opinion a piece of spiral armor around a cable does not have the same electrical characteristics as EMT or RMC around a set single conductors.

 

[ActionDave]

I agree with iwire.

Don does bring up an interesting point about the language being changed and the old version was clearer, but it's still clear enough to prohibit using one run of conduit and one run of MC.

 

[jeremy.zinkofsky]

I agree with iwire.

Don does bring up an interesting point about the language being changed and the old version was clearer, but it's still clear enough to prohibit using one run of conduit and one run of MC.

This is the exact conversation that i was trying to drum up and what I originally meant by "up for interpretation". The wording of 310.10(H)(3) was not clear enough, in my opinion.

A reference to Ch. 9 would have been nice as, while very useful, the NEC is not responsible for teaching electrical theory. As an engineer I have recieved a thorough education in such theory but an electrician may not have. To that regard, a qualified person who normally relies solely on the NEC for guidance may have been confused as to why 310.10(H)(3) makes sense. I can only speak for myself in saying that when I was an electrician I had no idea what reactance was and would have only speculated as to what it had to do with the electrical properties of a raceway.

 

[jumper]

If you used say PVC for one and RMC for another the current will not split evenly.

I did not know that. Thanks.

 

[iwire]

This is the exact conversation that i was trying to drum up and what I originally meant by "up for interpretation". The wording of 310.10(H)(3) was not clear enough, in my opinion.

Do cable armor and EMT have the same electrical characteristics?

I do not believe that question is up for interpretation I believe that the answer is a clear no.